1. Field of the Invention
The present invention relates to a deformable mirror device having a surface shape deformable by electrostatic force, and an apparatus for observing retina of an eye using such a deformable mirror device.
2. Related Art
In general, the apparatus for observing retina of an eye is an apparatus which irradiates retina of an eye to be examined with illumination light and receives and detects its image through an retina image forming optical system by using an image pickup device (for example, a CCD camera). For detecting and preventing a disease concerning eyes, it is desirable that the detection accuracy and resolution are as high as possible. Since an eyeball is not an ideal lens having no aberration, however, the eyeball has a wavefront aberration which becomes a factor of lowering the detection precision and resolution.
Therefore, a deformable mirror device which can vary its surface shape on the basis of information supplied from a controller is provided between the image pickup device which detects the image of the retina and the retina of the eye to be examined. The image of the retina of the eye to be examined reflected by the deformable mirror device is led to a wavefront sensor (for example, the Shack Hartmann sensor) to detect wavefront aberration. A control device applies voltages to electrodes of the deformable mirror device so as to reduce or eliminate the wavefront aberration on the basis of the detected wavefront aberration and causes an electrostatic force to act upon the deformable mirror to deform it. Owing to this deformation, an image having no wavefront aberration is obtained by the image pickup device.
Deformable mirror devices having a shape deformable by electrostatic sucking force is known (see, for example, FIG. 2 in JP-A-02-101402 (KOKAI)). The deformable mirror device shown in FIG. 2 in JP-A-02-101402 (KOKAI) has a structure consisting of a fixed electrode film 12 formed on an insulative substrate 11, a spacer part 18 having an opening in the center disposed on the fixed electrode film 12, a reflection film 17, a movable electrode film 16 and a SiO2 insulation film 14 stacked on the spacer part 18 so as to cover the opening, and a silicon substrate 13 having an opening in the center disposed on the stacked film. Therefore, the stacked film consisting of the reflection film 17, the movable electrode film 16 and the SiO2 insulation film 14 makes up a membrane part having its peripheral part fixed by the spacer part 18 and the silicon substrate 13, whose central part is deformed by electrostatic force between the fixed electrode 12 and the movable electrode film 16.
For observing retina photoreceptor cells, it is necessary for the deformation of the membrane part to follow eyeball movements such as rotation and translation. Thus, the membrane part is required to have high dynamic characteristics. The deformable mirror device having the shape deformable by electrostatic sucking force has a structure in which the fixed electrode and the membrane part face each other via the spacer part.
Letting F be electrostatic sucking force, ∈ be the dielectric constant (=8.85×10−12 F/m) of air, S be an electrode area, g be an initial distance (gap) between the fixed electrode and the membrane part, x be a deflection amount of the membrane part, and V be an applied voltage, electrostatic force is given by the following equation (1).
                    F        =                              1            2                    ⁢          ɛ          ⁢                      S                                          (                                  g                  -                  x                                )                            2                                ⁢                      V            2                                              (        1        )            
As the distance between the fixed electrode and the membrane part becomes smaller, the electrostatic sucking force becomes larger. If it is desired to drive the deformable mirror device by a low applied voltage, therefore, the distance must be very small.
In the meantime, the squeeze film effect that the time average pressure in a gap over one period becomes higher than that in the neighborhood by vibration in a relative vertical direction of the gap distance between two opposed faces is known. This squeeze film effect occurs between the fixed electrode and the membrane part and exerts a substantial influence upon the vibration characteristics of the membrane part. As a conventional method for reducing the viscosity coefficient C of the vibration system associated with the squeeze film effect, a measure has been taken by sealing the entire deformable mirror device including the fixed electrode and the membrane part in a reduced pressure environment. However, the reduced pressure sealing (reduced pressure packaging) requires a very high cost and there is a problem that leak of the gas inside the sealing brings about defective products, resulting in a bad cost efficiency.